Aromatic intermediates such as benzene, toluene and xylenes (“BTX”) may be obtained from petroleum naphtha, using a combination of processes to form and recover the desired aromatics. Catalytic reforming generally is the heart of an aromatics complex, producing a mixture of principally aromatics and paraffinic hydrocarbons to be processed further by some combination of aromatics extraction, dealkylation, transalkylation, disproportionation, adsorption or crystallization, isomerization and fractionation. The various steps were combined to address the issues of achieving high aromatics purity, balancing the product slate in favor of the relatively higher demand for benzene and xylenes, and dealing with the ethylbenzene contained in the mixed xylenes stream. Substantial improvements have been effected in individual processes contained in such aromatics complexes, particularly in catalytic reforming efficiency for aromatics production and in isomerization for conversion of C8 aromatics. Exemplary aromatics complexes are described in, for example, U.S. Pat. Nos. 8,609,922 and 8,431,758.
A common feature of many BTX aromatic complexes is one or more vent gas (or off-gas) streams. The vent gas streams comprise significant amounts of desirable aromatics components. The vent gas streams may comprise a vapor stream from a cold separator vessel, a vapor stream from a vent drum for a fractionation column, a vapor stream from a separator, or a vapor stream from a receiver associated with a stripper column. For example, many aromatics complexes include a stripper column for a transalkylation effluent. The overhead vapor from the column comprises a mixture of light ends (C3-hydrocarbons) and some aromatic hydrocarbons.
The above sources of waste gas can be split into categories: continuous and intermittent. The continuous sources comprise the receivers and vent drums while the intermittent sources include the separators and cold separators. The continuous sources constantly produce a vapor stream, while the production of a vapor stream from the intermittent sources depends on a number of factors, including processing conditions and feed quality.
Aromatic hydrocarbons that are recovered from the waste gas can be converted to benzene or paraxylene. If the aromatic hydrocarbons are left in the waste gas, the aromatic hydrocarbons will be used as fuel gas for heating. The benzene and para-xylene can be more than twice as valuable as the fuel gas. Therefore, it would be more desirable to recover the aromatics from these vapor streams instead of using the aromatics as less valuable fuel gas.
In order to recover the aromatics from the vapor streams associated with the continuous sources discussed above, the pressure on the columns is increased or the vent gas is compressed. The vent gas stream is then chilled in order to recover as much as benzene and heavier aromatic hydrocarbons as possible. This recovery process is typically performed at the highest off-gas system pressure.
However, a drawback of these is the necessary increased energy consumption required to compress vapor streams or refrigerant streams. Additionally, further energy may be wasted if the reboilers of the associated fractionation columns are now too hot to be heat integrated with another process stream. Therefore, since refiners are constantly seeking to improve yields and recovery, the recovery of the aromatic hydrocarbons from the vent gas is desired, but the energy consumption and loss are a drawback for the recovery.
With respect to the intermittent sources, it is believed that very little is done to attempt to recover the aromatic hydrocarbons from these sources of vent gas.
Accordingly, there remains a need for an effective and efficient process for recovering the aromatic hydrocarbons from a vapor stream.
It would be desirable to provide a process that also effectively and efficiently allows for the aromatic hydrocarbons to be recovered from the vent gas from an intermittent source.